The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Appliances, such as dishwashers, clothes washers and water heaters, for example, employ a heating element for heating water or other liquid that is used in the appliance. The heating element is immersed in the water to be heated. When the heating element is energized, it produces heat that is transferred to the surrounding water.
Such heating elements generally comprise a resistance heater that produces heat when an electrical current is passed through it. A typical tubular heating element comprises a coiled resistance wire extending coaxially along the length of an elongate metal sheath. An electrically insulating material having a relatively high thermal conductivity is used to fill the space between the coil and the inner wall of the sheath. The resistance wire is commonly made from metals such as Fe/Cr/Al or Ni/Cr. Granulated magnesium oxide (MgO) is one substance known to be suitable for serving as the filler material.
During the heating element's manufacturing process, the granulated magnesium oxide is introduced into the sheath. The sheath is subsequently subjected to a compression force, which causes the sheath to reduce in diameter, increase in length and compact the granulated magnesium oxide inside. In the compacted state, the magnesium oxide's dielectric and thermal conductive properties are improved. As a result of the compacting process, the heating element may be “partially compacted” (e.g., the diameter of the heating element is reduced by approximately 15% or less of its original diameter, such as from 0.375 in. to 0.334 in. (8.5 mm)) or “fully compacted” (e.g., the diameter of the heating element is reduced by approximately 15% or more of its original diameter, such as from 0.375 in. to 0.315 in. (8.0 mm)). Fully compacted heating elements are generally preferred over partially compacted heating elements due to performance and reliability advantages, such as increased efficiency of heat transfer from the resistance wire to the sheath and increased ability to manipulate or bend the heating assembly to fit particular applications, for example.
The heating elements of the type described also include a thermal protection device, such as a thermally-actuated cutoff switch or a thermally-actuated fuse. The thermal protection device allows current to pass to the resistance wire at normal operating temperatures, but it prevents or “cuts off” the current to the resistance wire if the temperature of the heating element exceeds a predetermined threshold temperature. The thermal protection device is typically embedded in the heating element adjacent to and in a thermally conductive relationship with, the resistance wire. This is accomplished via a metal terminal pin that is connected to the resistance wire on one end and the thermal protection device on the other.
During operation of the heating element, heat generated by the resistance wire is conducted through the terminal pin to the thermal protection device. In instances where the heating element approaches and/or exceeds the predetermined temperature, the thermal protection device cuts off the current to the resistance wire.
One condition under which the heating element may exceed the predetermined threshold temperature is a “dry start;” that is, when the heating element is energized but, the heating element is not immersed in liquid. When a dry start occurs, the heating element quickly heats up to a temperature beyond its normal operating temperature such that the heating element or the appliance in which it is installed may be damaged or rendered inoperable. Therefore, it is important for the thermal protection device of the heating element to react very quickly (e.g., less than 80 seconds) to cut off the current to the heating element when the predetermined threshold temperature is reached so as to eliminate or minimize any damage to the appliance or its components.
In order to achieve the desired reaction time in the thermal protection device, the efficient transfer of thermal energy from the resistance wire to the thermal protection device is desired. In this regard, it is important to securely fasten the terminal pin to the resistance wire. The construction of known water-immersed heating elements incorporates a metal terminal pin (usually made from steel) that is welded to the resistance wire. Even better heat transfer characteristics and reaction times, however, can be achieved with terminal pins that are made from copper, since copper has superior electrical and thermal conductivity as compared to steel. A copper terminal pin, though, is not easily welded to the heating element. This is so because the material composition of the resistance wire, e.g., Fe/Cr/Al or Ni/Cr, is not suitable for welding to copper without the use of advanced welding techniques, like laser welding or ultrasonic welding, for example, which generally are not considered to be cost-effective in this application. Consequently, construction of heating elements having a copper terminal pin has employed a connection method less robust than welding. There, the coiled resistance wire of the heating element is typically attached to the terminal pin by being “screwed onto” groves that are formed in the end of the terminal pin.
While marginally acceptable in the manufacture of partially compacted heating elements, the “screw on” connection method has proved less suitable for consistent and reliable production of fully compacted heating elements. In this regard, the forces applied to the heating element for compacting the magnesium oxide are known to degrade the physical and electrical connections between the resistance wire and the terminal pin. It is not uncommon in the manufacture of fully compacted heating elements that the resistance wire and the terminal pin become fully detached. In other cases, though the heating element and terminal pin do not completely separate during compaction, the resulting heating elements exhibit a high incidence of electrical arcing at the connection between the terminal pin and the resistance wire, thereby resulting in premature failure of the heating element.
Thus, there is opportunity for improvement of known water-immersed heating elements. For example, it is desirable to provide a heating element utilizing a copper terminal pin that provides a superior connection between the terminal pin and the resistance wire even after compaction.